Precision abrading machines are well known and are often utilized to abrade one or more surfaces of a workpiece to achieve a desired dimension. This is generally accomplished by using a process known as lapping which removes small, controlled amounts of material from the workpiece surface. One variety of abrading machine employs a fixed bridge. The bridge supports an upper lapping plate that is configured for rotation and vertical movement between a lower abrading position and an upper loading and unloading position. In the loading and unloading position, the workpiece can be loaded into the machine for subsequent lapping and thereafter unloaded when the desired dimension has been achieved. The distance between the loading and unloading positions, requiring the use of a relatively long shaft. This can result in a loss of rigidity and control during the abrading cycle, which in turn may result in reduced accuracy. In addition, machines of this type oftentimes utilize only a single cylinder to apply pressure from above during the abrading cycle. In some cases, however, the single cylinder configurations do not apply sufficient pressure for certain abrading processes.
In another variety of abrading machines, the lower lapping plate extends upward to meet a descending upper plate. That is, both the upper and lower plates move towards each other. Such arrangements, however, may have a problem associated with sealing gaskets and the creation of unwanted budding effects in the system during the lapping cycle.
Still other types of abrading machines utilize a sliding spindle and do not require the use of a long shaft. Such machines, however, are typically mounted on a single column. In one such known device, the upper plate is associated with an arm which is supported for vertical movement by a single column. The entire arm moves downward to position the upper plate. While effective for precision abrading, the apparatus is subject to an undesired cantilever effect during the abrading cycle. That is, when pressure is exerted during the lapping cycle, the arm and the column tend to act as a cantilever which results in loss of rigidity and control. This in turn may result in reduced accuracy. This problem, however, is substantially overcome through use of more recently developed dual column abrading machines.
One known abrading apparatus having a fixed lower plate utilizes a load cell or pressure sensor to detect the pressure applied to the workpieces by an upper rotatable and vertically movable plate. A displacement sensor detects the displacement of the rotating upper plate in a vertical direction as the vertical dimension of the workpiece or workpieces is reduced. The displacement sensor includes a probe which contacts a measurement surface on the upper plate assembly and forwards displacement measurements to a controller. This vertical displacement sensor is mounted on the base of the abrading apparatus and typically contacts a flat pad to provide a reference measurement. This reference measurement is then utilized as an input to a control system which calculates the current position of the upper plate to control the abrading process and determine when the desired workpiece dimension has been achieved. The abrading process is then terminated. This arrangement, however, suffers certain shortcomings. Since physical contact is made between the probe and the upper plate, the rotating upper plate may cause the displacement sensor to bounce or vibrate thereby negatively impacting the precision of the thickness measurements. Furthermore, the contact surface at which the measurement is taken may wear with time and use thus also negatively impacting precision. Additionally, since the electromagnetic displacement sensor measures the absolute distance to a reference surface and not the true thickness of the parts being machined, any increases or decreases in pressure may cause inaccuracies in the control system. Finally, any lateral shifting of the sensor with respect to the reference pad may introduce significant error into the measurement process.